Description of the Related Art
In the present era of heightened environmental concerns, electric-powered automobiles and similar vehicles are being considered as a highly desirable method of transportation. Existing battery technology, however, has limited the ability of vehicle manufacturers to produce marketable electric vehicles. Vehicle manufacturers desire a battery having sufficient stored energy capacity to allow the vehicle to travel an acceptable range between charging, while at the same time having acceleration and power characteristics similar to those of internal combination engines. Furthermore, vehicle manufacturers do not wish to sacrifice such marketable characteristics as vehicle size, appearance, and luxury features in order to reduce the load on the battery.
As noted above, existing battery technology is inadequate to meet these needs. Lead acid batteries, which are commercially available today, would by their very weight and size diminish the range capability of the resultant electric vehicle (EV).
Electrochemical batteries have been explored as possible alternatives to lead acid batteries, for example, sodium sulfur batteries and sodium metal chloride batteries. Sodium sulfur batteries utilize a sodium anode and sulfur cathode, separated by an ion-permeable electrolyte of beta" alumina. Sodium sulfur batteries have a high volumetric and gravimetric energy density compared to lead acid batteries.
To date, however, while sodium sulfur batteries may possess adequate stored energy capacity to allow a vehicle to travel long distances between charging, these batteries leave the EV underpowered. Prototype electric vehicles have displayed either relatively poor acceleration or poor range, depending on battery design preference. This result has placed an emphasis on simultaneously increasing the power and energy characteristics of these batteries.
The only way to accomplish this design task is by reducing the internal cell resistance. In order to reduce internal cell resistance in a sodium sulfur cell, the beta" alumina separator thickness must be reduced, thereby making it easier for sodium ions to migrate from the anode to the cathode. However, previous attempts to reduce separator thickness have resulted in excessive fragility, causing the beta" alumina to fracture easily, resulting in an impractical device.
At the same time, it remains desirable to maximize cell size (which is related to stored energy capacity), while keeping the overall battery as compact as possible. Related attempts to manufacture tubular battery cells have resulted in batteries with relatively thick beta" alumina separators (resulting in high internal resistance and reduced power), or small cells (resulting in low energy capacity and reduced vehicle range).